The circulation of fluids through open and closed systems has many applications in the arts, sciences and technology. Mechanical reciprocating pumps, centrifugal pumps, undulating tubes, thermal gradients and fans are all commonly used to move fluids. One particular application where the above listed circulating systems are often impractical is in bacteriological research where a sterile flask has a plug of cotton or other porous substance in the neck thereof and ambient air or other gas is allowed to pass through the cotton. For many reactions such as fermentation reactions the rate at which the air passes through the cotton is an important factor which determines the rate at which the reaction takes place.
A major shortcoming of the use of a cotton plug in the sterile shake flask is the very slow rate of gas exchange through the cotton plug. Consequently, the gas exchange through the cotton plug is rate limiting rather than the biological process in the bacteriological medium. Elaborate sterile gas pumping systems have been developed and used to increase the rate of air throughput. However, such systems are quite expensive, difficult to operate and maintain and provide a source of possible contamination. Some bacteriological processes are carried out under reduced pressure or elevated pressure and reactions are also carried out in the presence of a particular gas.
More particularly, the forced flow of gases has typically utilized mechanical compressors or other devices which can give rise to impurities caused by the necessary presence of lubricants. The need for moving gases in highly purified conditions has made most mechanical systems impractical. Furthermore, many gases are not compatible with the common materials of construction and thus can not be pumped by conventional devices. Still further, some processes require elevated temperatures or reduced temperatures. The design of systems for circulating air or other gases is made more difficult by the presence of such conditions.
Before gases used or generated in industrial processing may be discharged into the atmosphere, the gases are required, by law, to be depolluted of objectionable substances. A large part of the cost involved in the depollution process stems from the cost of physically moving the gases through complex systems.
The movement of solvents and solutes through semipermeable membranes has been studied extensively. The phenomena of reverse osmosis through a membrane is used in the desalination of sea water and other liquid purification processes.
When a solution, U, of solute, X in solvent Y, is placed on one side of a semipermeable membrane, M, which is permeable to the solvent, Y, but impermeable to the solute, X, and the pure solvent, Y, is placed on the other side of this semipermeable membrane, M, then an osmotic pressure, P.sub.c, develops in the solution U, such that: EQU P.sub.o =cRT EQ. (1)
Where "c" is the molar concentration of the solute X in the solution U; R is the universal gas constant and T is the absolute temperature. This equation for osmotic pressure was proposed by van't Hoff in 1887.
In the above model, when the solution, U, and the pure solvent, Y, exert the same hydrostatic pressure on the membrane, M, the differential external pressure on the membrane P.sub.e, is zero, and the osmotic pressure, P.sub.o, generated in this system produces a net flow rate, J.sub.o, of the solvent, Y, from the side containing the pure solvent through the semipermeable membrane into the side holding the solution, U, of solute, X, in solvent, Y. J.sub.o is the net flow rate of liquid through the membrane when the differential external pressure on the membrane, P.sub.e, is zero.
In the above model, if a differential external pressure on the membrane, P.sub.e, is exerted through the solution, U, such that P.sub.e is greater than the osmotic pressure, P.sub.o, then the pure solvent, Y, will flow in the reverse direction, from the solution, U, side of the semipermeable membrane into the pure solvent side of the semipermeable membrane. Large scale practical application of this is made in reverse osmosis where pure water is obtained from salt water by the use of a semipermeable membrane, pervious to water but impervious to salt. A very general interrelationship, characteristic of transport phenomena in general during irreversible thermodynamics, for fluxes, forces and their phenomonological coefficients was developed by L. Onsager.
In many instances it is desired to move a fluid (whether it be a gas or a solution) in which there is no concentration difference across the path of flow. In such cases no osmotic pressure, P.sub.o, exists. A system is thus needed which will move fluids where there is no concentration or composition change along the flow path.